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<tt>SWIG/Examples/perl5/value/</tt>
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<H2>Passing and Returning Structures by Value</H2>

<p>
Occasionally, a C program will manipulate structures by value such as shown in the
following code:

<blockquote>
<pre>
/* File : example.c */

typedef struct Vector {
   double x, y, z;
} Vector;

double dot_product(Vector a, Vector b) {
  return (a.x*b.x + a.y*b.y + a.z*b.z);
}

Vector vector_add(Vector a, Vector b) {
  Vector r;
  r.x = a.x + b.x;
  r.y = a.y + b.y;
  r.z = a.z + b.z;
  return r;
}
</pre>
</blockquote>

Since SWIG only knows how to manage pointers to structures (not their internal
representation), the following translations are made when wrappers are
created:

<blockquote>
<pre>
double wrap_dot_product(Vector *a, Vector *b) {
    return dot_product(*a,*b);
}

Vector *wrap_vector_add(Vector *a, Vector *b) {
    Vector *r = (Vector *) malloc(sizeof(Vector));
    *r = vector_add(*a,*b);
    return r;
}
</pre>
</blockquote>

The functions are then called using pointers from the scripting language interface.
It should also be noted that any function that returns a structure by value results
in an implicit memory allocation. This will be a memory leak unless you take steps
to free the result (see below).

<h2>The SWIG interface</h2>

Click <a href="example.i">here</a> to see a SWIG interface file that
wraps these two functions.  In this file, there are a few essential features:

<ul>
<li>A wrapper for the <tt>free()</tt> function is created so that we
can clean up the return result created by <tt>vector_add()</tt>
function.

<p>
<li>The %inline directive is used to create a few helper functions for creating new Vector
objects and to print out the value (for debugging purposes).
</ul>

<h2>A Perl Script</h2>

Click <a href="runme.pl">here</a> to see a script that uses these functions from Perl.

<h2>Notes</h2>

<ul>
<li>When the '<tt>-c++</tt>' option is used, the resulting wrapper code for the return value
changes to the following:

<blockquote>
<pre>
Vector *wrap_vector_add(Vector *a, Vector *b) {
    Vector *r = new Vector(vector_add(*a,*b));
    return r;
}
</pre>
</blockquote>

Similarly, it would be a mistake to use the <tt>free()</tt> function from C++.  A safer 
approach would be to write a helper function like this:

<blockquote>
<pre>
%inline %{
   void delete_Vector(Vector *v) {
       delete v;
   }
%}
</pre>
</blockquote>

<p>
<li>If you use proxy classes and are careful, the SWIG generated wrappers can automatically
clean up the result of return-by-reference when the scripting variable goes out of scope.

<p>
<li>Passing parameters by value like this really isn't the best C programming style.
If possible, you might change your application to use pointers.

<p>
<li>Similar translations are made when C++ references are used.


</ul>

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